One of the main characteristics providing the possibility of use of atomic emission spectral (AES) analysis for solution of particular analytical problems is sensitivity (detection limit) defined by intensity of AES lines of impurity atoms in the specimen (sample) being analyzed.
At present, primarily atomic-emission and atomic-absorption spectroscopy methods are used for determining the impurities concentration in liquid media with detection limit of lower than 1 ppm [Journal of Analytical Chemistry. 2011, vol. 66, No. 9, pp. 900-915]. To determine the concentration of impurities with the use of analytical instruments implementing such methods, one must perform sampling, feed appropriate amount of inert or combustible gas in the course of each analysis, and provide power supply, generally higher than 1 kW. The use of such analyzers requires provision of certain conditions, availability of highly skilled operating personnel and material expenses, which complicates the application of such methods in mobile and self-contained versions.
There is a prior art method of emission analysis based on an electric discharge directly in the liquid being analyzed (EDBC i.e. electric discharge corresponding to the boiling in a channel). [B. Zuyev, V. Yagov, M. Getsina, B. Rudenko, Journal of Analytical Chemistry, 2002, vol. 57, No. 10, pp. 1072-1077]. The method uses for EDBC initiation a two-electrode electrolytic cell filled with electrolyte solution and having spaces separated by a dielectric partition (membrane). The membrane has an opening with a diameter of about 1 mm and a length equal to the membrane thickness, which forms a channel for liquid passing between two spaces of the cell. High-voltage electrodes are arranged on different sides of the membrane. Voltage of (1.2-2.8) kV, depending on the composition of the analyzed solution and on the channel size, is applied to the cell electrodes. As a high-voltage electric circuit is closed in the channel where current density is much higher than in the rest part of the vessel, the liquid boils due to ohmic heat generation, and a steam bubble is formed. Then the channel is blocked by the steam plug, and a gas discharge accompanied by light emission arises between the plug walls. Optical emission intensity of atoms in the EDBC torch serves as an analytical signal for further recording of the emission spectra by means of a spectrometer.
The drawback of the above-described method is that the emission spectra intensity detected for the majority of elements, including alkaline-earth metals, is low, significantly lower than for alkaline metals (according to the inventors' data, the detection limit was 0.05 ppm for sodium, 1.5 ppm for calcium and 5 ppm for magnesium).
The prior art method most similar to the proposed invention is a method for emission analysis of elemental composition of liquid media based on local electric discharge (LED) in liquid [Patent RU 2368895, GO1N 27/62, 27.09.2009]. The method also includes electric current flow through the liquid under analysis by means of electrodes separated by a partition (membrane) made of dielectric material and having a diaphragm opening therein. To initiate a local electric discharge (LED) in the diaphragm opening, concentration of electric power dispersed in a unit of the liquid volume sufficient for obtaining and sustaining a stable discharge is provided in the diaphragm opening where a current-carrying channel is formed. This takes place due to smaller volume of liquid participating in the discharge (diameter of the diaphragm channel is 0.1 mm) and higher discharge voltage (up to 15 kV). One of the electrodes (a current-carrying member) is located near the diaphragm opening, directly in the area of the discharge. LED is initiated after polarization of the above-mentioned electrode by current of the same polarity as that of LED current. The polarization current intensity is lower than that required for LED inception but sufficient for efficient mass transfer between this electrode and ions of the liquid being analyzed (in practice, 0.1-0.3 of LED current). Emission spectra of elements contained in the liquid excited by LED are recorded by means of a spectrometer.
The drawbacks of the method include presence of emission lines of the electrode material in the recorded spectrum of analyzed liquid and spatial instability of emission detected by the spectrometer due to the discharge movement over the electrode surface. Both drawbacks are caused by the electrode location in the area of the discharge, which leads to gradual destruction (sputtering). This results in instability of the detected spectra and, therefore, adversely impacts reproducibility of the measurement results and long-term operation reliability of the device.